A gigabit channel is a communication channel with a total data throughput of one gigabit per second. A gigabit channel typically includes four unshielded twisted pairs (hereinafter “UTP”) of cables (e.g., Category-5 twisted pair cables) to achieve this data rate. I.E.E.E. Standard 802.3 ab, herein incorporated by reference, describes the specifications for 1000BASE-T twisted-pair gigabit Ethernet. For signal transmission, various types of output stages can be used to drive resistive loads, such as UTPs, for data transmission in accordance with Ethernet network protocols, such as gigabit Ethernet.
For purposes of illustration, FIG. 1A illustrates a simple transmitter 100 for transmitting a differential output current signal, IOUT. The transmitter 100 includes a first current source 102 configured to generate the positive component signal of the differential output current signal. A second current source 104 is configured to generate the negative component signal of the differential output current signal. The transmitter 100 is coupled to an interface circuit 110 for interfacing the transmitter 100 to a UTP 120. The interface circuit 110 can include resistors 112 and 114 arranged in series with a common-mode voltage VCM 116 located between them. The resistors 112 and 114 are arranged in parallel across the primary windings of an isolation transformer 118, with the secondary windings coupled to the UTP 120. The isolation transformer 118 includes a center tap on the primary windings with a DC center tap voltage, VCT 125. In differential mode, IOUT=IOUT+−IOUT−. The magnitude of IOUT depends on the symbol to be transmitted and can vary, for example, from −40 mA to 40 mA (e.g., in 1000BASE-T and 100BASE-TX) and from −100 mA to 100 mA (e.g., in 10BASE-T). A bias or quiescent current, IBIAS, is supplied by bias current supply 127 to bias the first and second current sources 102 and 104, as discussed below.
In 100BASE-T, for example, three transmit symbols are used: {−1, 0, 1}, where a positive pulse represents a “+1,” a negative pulse represents a “−1,” and the signal represents “0” otherwise. For purposes of illustration, FIG. 1B illustrates a transmit signal, VTX 130, for transmitting the symbol sequence {0, +1, 0, −1}. Several different classes of operation exist for transmitting such signals.
FIG. 1C illustrates an example of class A operation. In class A operation, the output devices conduct for the entire cycle of the output signal. In other words, both output devices conduct continuously for the entire cycle of the output signal. Class A operation typically biases drivers to a certain (large) quiescent or bias current, IBIAS, e.g., IBIAS=40 mA. For purposes of illustration, the transmitter 100 can drive a differential current 140 of 40 mA for class A operation. The center tap current of the transformer 118 will therefore be constant at 40 mA. Consequently, the corresponding common-mode current will not change, resulting in substantially noiseless operation of the transformer 118.
FIG. 1D illustrates an example of class B operation. In class B operation, the output devices conduct for approximately fifty percent of the cycle of the output signal. In other words, each output device is only turned on when it is driving a signal, otherwise it is turned off. Due to this operation, class B operation provides higher efficiency than class A operation, but poor linearity around the crossover region, due to the time it takes to turn one device off and the other device on. The bias current for class B operation is generally very small, being close to zero (and at zero in the ideal case). For purposes of illustration, the transmitter 100 can also drive a differential current 140 of 40 mA for class B operation. However, unlike in class A operation, the center tap current of the transformer 118 will be a transient current of 40 mA for transmitting symbols “+1” and “−1.” Consequently, the common-mode current will change on the center tap, thereby inducing electro-magnetic interference (EMI) in the transformer 118. Such EMI will affect transmission of the output signal.
FIG. 1E illustrates an example of class A-B operation. In class A-B operation, the output devices conduct for greater than fifty percent, but less than one hundred percent, of the cycle of the output signal. Both output devices, then, conduct simultaneously for a portion of the cycle of the output signal. In class A-B operation, the drivers are carefully biased just above their fully off state so that the transition between drivers is smoother, thereby causing the output devices to conduct for more than half of, but less than the entire, cycle. Class A-B operation requires more bias current than in equivalent class B operation, but less bias current than in equivalent class A operation. For purposes of illustration, the transmitter 100 can also drive a differential current 140 of 40 mA for class A-B operation. As in class B operation, the center tap current of the transformer 118 will be a transient current for transmitting symbols “+1” and “−1.” Consequently, the common-mode current will change on the center tap, inducing EMI in the transformer 118, and again affecting transmission of the output signal.
Consequently, there is a need for a transmission scheme that can make transformer behavior more linear, as well as reduce EMI in the transformer.